ButtonProgressWidget: Make hold_callback optional (#8940)
ButtonProgressWidget: Make hold_callback optional (#8940)

We don't need one in FrontLightWidget ;).

Fix https://github.com/koreader/koreader/issues/8913#issuecomment-1074915143
Frenzie/koreader,NiLuJe/koreader,Frenzie/koreader,koreader/koreader,poire-z/koreader,koreader/koreader,poire-z/koreader,NiLuJe/koreader